The fact that different part numbers are used is quite a debate for many applications.

From what we have been told, Eibach USA and Germany/Japan operate as separate entities, and as a result have different part numbers. Testing is done independently as well. This is because there are quite a lot of cars that come with different suspensions depending on region, such as the Scion FRS/BRZ/FT86 (different spring rates across each model between GER/AUS/USA).

The second part may also have to do with TUV testing, which is not a requirement in USA (this is yet to be 100% confirmed, we are waiting for a response on this).